Engine air-fuel ratio can be controlled to provide improved catalyst performance, reduce emissions and improve engine fuel efficiency. Specifically, systems to control air-fuel ratio in engine cylinders may include monitoring of exhaust gas oxygen concentration at an exhaust gas sensor and adjusting fuel and/or charge air parameters to reduce air-fuel ratio variation, minimize degradation of exhaust catalyst and improve engine performance.
An example of an engine air-fuel ratio control system and method is provided by Makki et al in U.S. Pat. No. 7,000,379. Therein an inner feedback control loop is used to control engine air-fuel ratio based on input from a first exhaust sensor coupled upstream of an exhaust catalyst, and an outer feedback control loop is used to modify the air-fuel ratio provided to the inner feedback control loop to maintain the output of a second exhaust sensor (coupled on the exhaust catalyst) within a predetermined range of a desired reference value. The catalyst model determines changes in catalyst dynamics based on input from the second exhaust sensor.
However, when using such an engine air-fuel ratio control system, factors such as the geometry of the exhaust system, and a location and sensitivity of the exhaust gas sensors may create discrepancies in a measured air-fuel ratio. For example, an exhaust gas sensor coupled upstream of an engine exhaust system receiving exhaust from multiple cylinders may bias sensor readings toward output of cylinders close to the exhaust gas sensor more than output from cylinders afar. Consequently, it may be difficult to determine cylinder to cylinder air-fuel ratio imbalance in engines with multiple cylinders. Further, poor exhaust mixing at the exhaust gas sensor may create further discrepancies in the measured air-fuel ratio and make it difficult to correct cylinder air-fuel ratio imbalance.
In other engine systems, cylinder air-fuel ratio imbalance can be monitored using methods based on crankshaft acceleration. However, transient changes in torque demand (such as from various engine accessory loads) and purge errors may affect the learning of cylinder air-fuel ratio imbalance.
In view of the above, the inventors herein have developed a method for determining air-fuel ratio imbalance among cylinder groups. In one example, a method comprises: during a deceleration fuel shut-off (DFSO), sequentially firing cylinders of a cylinder group, each cylinder fueled with a fuel pulse width selected to provide an expected air-fuel deviation; and indicating an air-fuel ratio variation for each cylinder based on an error between an actual air-fuel deviation from a maximum lean air-fuel ratio during the DFSO relative to the expected air-fuel deviation. In one example, the learning may be performed based on the air-fuel deviation estimated at a heated exhaust gas sensor. In this way, learning an air-fuel ratio imbalance in each engine cylinder may be improved while minimizing issues related to sensor sensitivity and exhaust mixing.
For example, responsive to a first rich air-fuel variation in a cylinder (wherein an actual air-fuel ratio is richer than an expected air-fuel ratio), a controller may learn a first air-fuel error and during subsequent operation, the fueling of the cylinder may be enleaned as a function of the first air-fuel error. Likewise, responsive to a second lean air-fuel variation in a cylinder (wherein an actual air-fuel ratio is leaner than an expected air-fuel ratio), the controller may learn a second air-fuel error and during subsequent operation, the fueling of the cylinder may be enriched as a function of the second air-fuel error. By determining cylinder air-fuel imbalance based on air-fuel variation and adjusting fueling in a cylinder based on the air-fuel error, cylinder air-fuel ratio variations may be reduced while minimizing issues related to sensor sensitivity and exhaust mixing.
The approach described here may confer several advantages. For example, the air-fuel ratio error is learned when a single cylinder in each cylinder bank of an engine is firing while the remaining cylinders are deactivated, allowing better detection of air-fuel ratio imbalance among cylinder groups. Consequently, the approach ensures reduced emissions and improved fuel efficiency. Furthermore, by learning cylinder air-fuel ratio imbalance based on sensor readings at a downstream exhaust gas sensor, issues related to sensor location and sensitivity may be further reduced while minimizing error due to poor exhaust mixing.
The above discussion includes recognitions made by the inventors and not admitted to be generally known. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.